As mobile telecommunications networks are used more and more in everyday life, many users now have multiple user equipments, UEs. For example, a user may have a tablet, a personal mobile phone, a business mobile phone, etc. In order to simplify the experience for such a user, IP Multimedia Subsystem, IMS, networks provide functionality for having a single SIP address for a user be directed to multiple UEs (e.g. a user may have a single telephone number which is associated with multiple UEs). When an INVITE request is sent to the SIP address of the user, the network will fork the INVITE request to each of the UEs associated with the address. The session will be established with whichever of the UEs accepts the INVITE request first by responding with a 200 OK.
Similarly, many UEs are now capable of handling multiple simultaneous sessions, using features such as call waiting, ad-hoc conference calls, or split-screen video calling. However, if a user already has a session in progress on one of their UEs, the IMS will still offer all incoming session requests to all UEs registered for the user. As such, the call may be accepted by a UE other than that involved in the first session, which prevents the UE involved in the first session from making use of the simultaneous session handling features. Furthermore, the user may struggle to use both UEs involved with sessions at once, or the second session may not be answered by the user (e.g. in the case where a user has left one of their UEs at home, and someone in the household hears the UE ringing and answers it on their behalf).